1. Field of the Invention
This invention relates to curable epoxy resins having increased adhesive strength; and, more particularly, to amine cured epoxy resins containing certain polyether diamide additives.
2. Prior Art
Epoxy resins constitute a broad class of polymeric materials having a wide range of physical characteristics. The resins are characterized by epoxide groups which are cured by reaction with certain catalysts or curing agents to provide cured epoxy resin compositions with certain desirable properties. One such class of curing agents are generally the amines. The most commonly used amine curing agents are aliphatic amines such as diethylenetriamine, triethylenetetramine and the like and/or polyoxyalkylene polyamine; such as polyoxypropylenediamines and triamines.
Epoxy resin compositions having improved physical properties are obtained by employing polyoxyalkylenepolyamines, and polyoxyalkylenediamines in particular, as curing agents. It is common to employ with such epoxy resin compositions a co-curing agent such as those described in U.S. Pat. No. 3,549,592.
It is known to use polyamides as epoxy curing agents. Simple amides such as acetamide, benzamide and adipamide have been used, but low activity and/or solubility require use of basic catalysts. The advantages and disadvantages of polyamides as curing agents is discussed in Lee, Henry and Neville, K. Handbook of Epoxy Resins, McGraw Hill Book Co., New York, 1967, but, generally, the hydrogen of the primary or secondary amides are weakly reactive with epoxy groups.
It has now been found that a specific diamide terminated polyoxyalkylene material having a molecular weight of from about 2000 to about 3000, when employed as an additive to amine cured epoxy resins provides cured compositions exhibiting outstanding strength of adhesion. Specifically, epoxy resins incorporating these additives, upon curing with an amine, provide a material with high tensile shear strength and superior adhesion to substrates.
The results of incorporating the additives of the instant invention are unexpectedly superior over similar type compounds of lower molecular weight. The cured epoxy resin compositions of the instant invention are useful as coatings, castings, sealants and especially adhesives.